We wish to use MAD phasing techniques to solve the crystal structure of a proteinase inhibitor (CHFI) which shows particular selectivity for the human blood coagulation Factor XIIa. Factor XII is part of the "contact system" of plasma, which after contacting appropriate surfaces, can lead to blood coagulation, activation of the complement system, fibrinolysis, or inflammation. The specificity of CHFI for factor XIIa makes it a useful reagent of potential importance in regulating any or all of these systems. The protein contains 127 residues and is readily crystallized. We have searched for heavy atom derivatives for use in multiple isomorphous replacement phasing attempts, but we have not found any useful derivatives. Molecular replacement methods using NMR structures of a related inhibitor have not been successful in solving this structure. The protein can be expressed in E. coli and grown with selenium-methionine substituting for the three methionines in the expressed protein.